(i) Field of the Invention
The present invention relates to a semiconductor laser, and more specifically, it relates to a semiconductor laser for use in an optical communication system.
(ii) Description of the Prior Art
With the progress of an optical communication technique, an applicable field of a semiconductor laser is rapidly spreading from a trunk transmission network to systems such as a subscriber system, LAN and data link. Particularly, in order to realize a total optical communication of from a communication center to each home, much attention has been paid to a passive double star (PDS), a passive optical network (PON) and the like where an original signal is distributed by an optical star coupler which is a passive part, for the reason why they are inexpensive. In these systems, a laser has been desired by which a high output operation can be carried out at a low drive current at a high temperature and a loss by the distribution of the original signal can be compensated. Therefore, there has been employed a method which comprises introducing a strained MQW structure into an active layer to change a band structure and to thereby increase a gain.
FIG. 6 shows a band structure of an active layer which is constituted of a strained MQW layer 4 comprising a well layer 11 and a barrier layer 12 formed by MOVPE growth, and a single SCH layer 5 having the same composition as in the barrier layer 12 (IEEE Photonics Technology Letters, Vol. 4, No. 9, September 1992, p. 954-957). In order to operate this semiconductor layer at a high temperature at a high output, it is necessary to increase an amount of light (an optical confinement coefficient) which is confined in the well layer, and so the thickness of an SCH (a separate confinement heterostructure) layer 5 is as thick as 100 nm. However, if the SCH layer is thick, there occur the deterioration of a carrier transportation efficiency by the recombination of carriers running through the SCH layer and the increase of an optical loss by free carrier plasma oscillation, so that the efficiency of the semiconductor laser deteriorates inconveniently.